Field of the Invention
The present invention relates to a capacitive discharge ignition apparatus chiefly employed in an internal combustion engine.
Background Art
Issues on environmental preservation and fuel depletion are being raised recently and actions toward these issues are urgently necessary in the auto industry. One example of such actions is ultra-lean combustion of an engine using a stratified mixture, so-called stratified lean combustion. Stratified lean combustion is a technique to burn a combustible air-fuel mixture generated only in a partial region within a combustion chamber, that is, a region in the vicinity of a sparking plug. With this technique, an intake loss can be reduced and a coefficient of thermal expansion can be enhanced.
Because fuel is collected only in the vicinity of the sparking plug by the stratified lean combustion, a method using a flow of air called a swirl flow is used extensively. This method takes an advantage of the nature of air that flows toward the center of a swirl. By disposing a sparking plug at the center of a swirl and allowing fuel to flow on the swirl, it becomes possible to collect the fuel in the vicinity of the sparking plug, thereby enabling the stratified lean combustion.
Accordingly, there is a possibility with the stratified lean combustion that the combustible mixture is not distributed homogeneously, and a spark discharge over a long period is required from the viewpoint of firing opportunity. A concentration of the mixture is not homogeneous, either, and in these circumstances, a fume leakage caused by carbon deposits on the sparking plug readily occurs. In view of the foregoing, a high secondary current is necessary for the stratified lean combustion to generate a spark discharge in a reliable manner even in circumstances where an energy leakage pathway has been formed.
In response to this necessity, an ignition apparatus described in Patent Document 1 has been proposed. The ignition apparatus described in Patent Document 1 uses a capacitive discharge ignition method to give rise to a breakdown between electrodes of an ignition plug. By supplying energy intermittently to a primary end of the sparking coil from a coil having pre-stored the energy to maintain a following inductive discharge, an AC spark discharge is generated continuously between the electrodes of the sparking plug. Both a high initial secondary current and a spark discharge over a long period can be thus achieved.
Patent Document 1: Japanese Patent No. 4497027
Meanwhile, strong eddying flow and current of the combustible mixture develop in the vicinity of the sparking plug and these eddying flow and current make it difficult to start and continue a spark discharge for fuel ignition. The spark discharge is a phenomenon that molecules between the electrodes are turned into plasma by a action or the like with a high voltage applied between the electrodes and a current is flowed through the plasma. There is, however, a phenomenon (blow-off phenomenon) that molecules turned into plasma per se are flowed by the strong current described above or the plasma disappears with cooling. It therefore becomes difficult to start a spark discharge and should a discharge be started, the spark discharge is interrupted when a path (plasma) through which to flow the current disappears.
The ignition apparatus described in Patent Document 1 has a problem that the blow-off phenomenon readily occurs during an operation on a low discharge current. In other words, because a discharge current per se of the spark discharge contributes to generation of plasma, which serves as a path, when the discharge current is large, the current itself is able to repair the path even when the path is blown off. The path is therefore seldom interrupted and the blow-off phenomenon hardly occurs. In contrast, when the discharge current is small, plasma generated by the current itself is too small to repair the path quickly enough. The path is therefore interrupted easily and the blow-off phenomenon readily occurs.
Hence, once the discharge is interrupted, it becomes impossible to give rise to a breakdown again between the electrodes of the sparking plug. The spark discharge is therefore interrupted and ends at this point.
In circumstances where there is a fume leakage pathway between the electrodes of the sparking plug, an initial breakdown is possible due to the capacitive discharge method. However, when a spark discharge current thereafter is small, a large proportion of energy leaks through the leakage pathway. Hence, as with the description above, there is a case where the spark discharge cannot be maintained, causing a firing opportunity to be missed.
In addition, the ignition apparatus of Patent Document 1 proposes a scheme equipped with DC-to-DC converters separately used for a normal operation and stratified lean combustion and generating a further larger discharge current. To achieve this scheme, however, a further larger DC-to-DC converter and a further larger energy storing coil are necessary. This proposed scheme therefore has problems of heat generation and a large size of the product.